Filling a 2d array with preset characters using loops?

Does anyone know how to fill this 2d array with a loop??

String s = "AABBCCDDEEFF"

String input[][];
input = new String [4][3];

input [0][0]= s.substring(0, 1); 
input [0][1]= s.substring(1, 2); 
input [0][2]= s.substring(2, 3); 
input [1][0]= s.substring(3, 4); 
input [1][1]= s.substring(4, 5); 
input [1][2]= s.substring(5, 6); 
input [2][0]= s.substring(6, 7); 
input [2][1]= s.substring(7, 8); 
input [2][2]= s.substring(8, 9); 
input [3][0]= s.substring(9, 10); 
input [3][1]= s.substring(10, 11); 
input [3][2]= s.substring(11, 12);  

This is the way that i did it before but i need to make it more efficient. I have tried the loops thing and have asked some friends (beginners at java like myself) but it just doesnt work. I think the main problem is that i dont know how to pick out characters from the string to place in the array. Any help is appretiated! Also i dont really get the nested loop thing which im pretty sure you need to get the array position you want to fill?

Thanks in advance!!